Field of the Invention
This invention relates to light emitting diode packages and displays utilizing light emitting diode packages as their light source.
Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
Technological advances over the last decade or more has resulted in LEDs having a smaller footprint, increased emitting efficiency, and reduced cost. LEDs also have an increased operation lifetime compared to other emitters. For example, the operational lifetime of an LED can be over 50,000 hours, while the operational lifetime of an incandescent bulb is approximately 2,000 hours. LEDs can also be more robust than other light sources and can consume less power. For these and other reasons, LEDs are becoming more popular and are now being used in more and more applications that have traditionally been the realm of incandescent, fluorescent, halogen and other emitters.
In order to use an LED chip in conventional applications it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical two-pin LED package/component 10 illustrated in FIG. 1, a single LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup 13 may be filled with an encapsulant material 16 and a wavelength conversion material, such as a phosphor, can be included in over the LED chip or in the encapsulant. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly can then be encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to direct or shape the light emitted from the LED chip 12.
A conventional LED package 20 illustrated in FIG. 2 may be more suited for high power operations which may generate more heat. In the LED package 20, one or more LED chips 22 are mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate or submount 23. A metal reflector 24 mounted on the submount 23 surrounds the LED chip(s) 22 and reflects light emitted by the LED chips 22 away from the package 20. The reflector 24 also provides mechanical protection to the LED chips 22. One or more wirebond connections 21 are made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The mounted LED chips 22 are then covered with an encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflector 24 is typically attached to the carrier by means of a solder or epoxy bond.
FIG. 3 shows another LED package 30 comprising a casing 32, and a lead frame 34 at least partially embedded in the casing 32. The lead frame 34 is arranged for surface mounting of the package 30. Portions of lead frame 34 are exposed through a cavity in the casing 32 with three LEDs 36a-c mounted on part of the lead frame 34 and connected to other parts of the lead frame by wire bonds 38. Different types of LEDs 36a-c can be used, with some packages having red, green and blue emitting LEDs. The package 30 comprises a pin-out structure with six pins 40 and the lead frame is arranged so that the emission of each of the LEDs 36a-c can be controlled independently by a respective pair of the pins 40. This allows the package to emit a variety of color combinations from the LEDs 36a-c. 
Different LEDs packages, such as those shown in FIGS. 1-3, can be used as the light source for signs and displays, both big and small. Large screen LED based displays (often referred to as giant screens) are becoming more common in many indoor and outdoor locations, such as at sporting arenas, race tracks, concerts and in large public areas such as Times Square in New York City. With current technology, some of these displays or screens can be as large as 60 feet tall and 60 feet wide. As technology advances it is expected that larger screens will be developed.
These screens can comprise thousands or hundreds of thousands of “pixels” or “pixel modules”, each of which can contain one or a plurality of LED chips or packages. The pixel modules can use high efficiency and high brightness LED chips that allow the displays to be visible from relatively far away, even in the daytime when subject to sunlight. In some signs each pixel can have a single LED chip, and pixel modules can have as few as three or four LEDs (such as one red, one green, and one blue) that allow the pixel to emit many different colors of light from combinations of red, green and/or blue light. The pixel modules can be arranged in a rectangular grid that can include hundreds of thousands of LEDs or LED packages. In one type of display, the grid can be 640 modules wide and 480 modules high, with the size of the screen being dependent upon the actual size of the pixel modules. As the number of pixels increases, the interconnect complexity for the display also increases. This interconnect complexity can be one of the major expenses for these displays, and can be one of the major sources of failure during manufacturing and during the operational lifetime of the display.